Semiconductor devices and liquid crystal display devices are produced by the so-called photolithography technique in which a pattern formed on a mask is transferred onto a photosensitive substrate. The exposure apparatus, which is used in the photolithography step, includes a mask stage for supporting the mask and a substrate stage for supporting the substrate. The pattern on the mask is transferred onto the substrate via a projection optical system while successively moving the mask stage and the substrate stage. In recent years, it is demanded to realize the higher resolution of the projection optical system in order to respond to the further advance of the higher integration of the device pattern. As the exposure wavelength to be used is shorter, the resolution of the projection optical system becomes higher. As the numerical aperture of the projection optical system is larger, the resolution of the projection optical system becomes higher. Therefore, the exposure wavelength, which is used for the exposure apparatus, is shortened year by year, and the numerical aperture of the projection optical system is increased as well.
If the patterns formed on the mask have any defects or distortions, such defects or distortions may result in lower exposure precision. For example, the patterns cannot be transferred to desired positions, or the shapes of the patterns may be transferred incorrectly. A pellicle is often used with a mask. A pellicle is a thin transparent layer that may be stretched over a frame above the surface of a reticle. Pellicles are used to block particles from reaching the patterned side of a reticle surface. Although particles on the pellicle surface are out of the focal plane and should not form an image on the wafer being exposed, it is still preferable to keep the mask surfaces as particle-free as possible. For certain types of lithography (e.g., extreme ultraviolet (EUV) lithography), however, pellicles are not used. Because the EUV masks are not covered, they are prone to particle contamination, which may cause defects in a lithographic process. Particles on EUV masks are one of the main sources of imaging defects. Inspection and cleaning of a EUV mask before moving the mask to an exposure position can be an important aspect of a mask handling process. Masks are typically cleaned when contamination is suspected, as a result of inspection, or on the basis of historical statistics.
A method of inspecting masks uses images obtained by scanning electron microscopes (SEM) is well-known. It is a common practice to acquire an SEM image of a high-magnification SEM image in a restricted narrow field of view with a high-magnification. In addition, if a wide field of view is scanned by an ordinary critical dimension SEM, various kinds of aberrations such as the astigmatism, the field curvature, and the distortion inevitably take place. These kinds of aberrations must be corrected, dynamically, in a simultaneous manner with the scanning action.
To obtain SEM high quality images with high resolution a high voltage bias on the mask should be used. In this case a special mask holder is used for handling a mask both within the inspection tool and out of the tool.